1. Field of the Invention
The invention relates to an optical non-contact method employing the luminous section principle to read the three-dimensional shape of a profile. It finds one particularly pertinent application in reading the shape of the inside edge of a spectacle frame rim, known as the bezel. The invention also relates to a device for implementing the method when applied to reading a spectacle frame bezel.
2. Description of the Prior Art
During the fabrication of a pair of spectacles, in order to be able to mount the lenses in the frame, it is necessary to adapt the outside edge of each lens to fit the inside edge of the corresponding rim of the frame, usually called the bezel. A numerically controlled grinding machine is usually employed for this purpose and adapts the outside edge of each lens to the shape of the bezel into which it must be crimped.
To perform numerically controlled grinding in this way, it is necessary to have a numerical model of the three-dimensional shape of the bezel concerned.
At present, the three-dimensional shape of a bezel is acquired by means of a contact-type measuring device in which a feeler associated with a measuring head rotating about the central axis of the frame rim comes into physical contact with the bezel and slides along the whole of its periphery. However, this contact-type measurement is not entirely satisfactory, for two main reasons. First of all, the feeler, winch is urged at all times against the bezel, can in some cases cause deformation of the frame rim, falsifying the measurements. Secondly, the mechanical system guiding the feeler cannot achieve complete acquisition of the profile of the bezel over the whole of its perimeter in an acceptable time, i.e. in less than one minute. This precision mechanical system is also relatively costly, although it can only provide an acceptable level of accuracy with sufficient radial movement of the feeler. The resulting fabrication and maintenance problems are detrimental to cost.
To remedy these drawbacks, a number of non-contact optical methods and devices have previously been proposed for reading the three-dimensional shape of the bezel.
Thus the documents FR 2 679 997 and FR 2 713 758 propose a non-contact optical method of reading the shape of a spectacle frame rim bezel using a narrow light beam (rectilinear coherent laser beam) to illuminate certain characteristic points of the bezel and an optical sensor with a matrix of CCD photosensors to register the impact of the beam at each of those points. The spatial coordinates of each of the points illuminated in this way are then computed by triangulation from the position, as acquired by the sensor, of the image of the point of impact of the beam on the frame and the respective spatial configurations of the laser beam and the sensor. To enable illumination and reading of points over the full height of the bezel, the laser beam, to be more precise its source, can move along the axis of the rim concerned of the frame, i.e. in practice vertically.
The documents WO 98/45664 and WO 00/03839 propose a similar type of reading method in which the light beam, instead of taking the form of a rectilinear narrow beam, diverges in a plane so that it intersects the bezel transversely. In this method, known as the luminous section method, the sensor receives the image of the trace (or luminous section) of the plane light beam on the bezel. Thus each optical reading does not relate to only one point on the bezel, but to a complete cross section of the bezel.
The above luminous section method is described in detail in the following document:
xe2x80x9cA perspective, on range finding techniques for computer visionxe2x80x9d, R. A. Jarvis, IEEE transactions on pattern analysis and machine intelligence, Vol. PAMI-5, No 2 March 1983.
It essentially entails scanning the profile with a plane light beam intersecting the profile transversely and simultaneously reading the trace of the plane light beam at a series of positions along the profile using optical receiver means having an optical pointing axis at a constant non-zero pointing angle to the light beam. Finally, a programmed computer deduces the three-dimensional shape of the profile from the readings effected at the various positions.
The documents WO 98/45664 and WO 00/03839 propose non-contact optical devices for reading the three-dimensional shape of the inside edge, referred to as the bezel, of a spectacle frame rim, which devices implement the above method and include a support for the frame rim and a read head which rotates relative to the support about a rotation axis and with which is associated a sensor responsive to its angular position relative to the support, the read head including emitter means adapted to project a plane light beam intersecting the bezel transversely and optical receiver means adapted, regardless of the angular position of the read head relative to the frame support, to read the trace of the plane light beam on the bezel along an optical pointing axis at a constant non-zero pointing angle to the light beam.
It is therefore clear that, compared to the method of reading points previously cited, the above luminous section method and the device for implementing it have the advantage of reading the complete section of the bezel over its full height each time that the optical receiver means capture an image and without it being necessary to provide for each angular position of the read head any vertical displacement of the laser beam to scan of the section concerned of the bezel transversely.
However, whichever type of light beam is used (rectilinear narrow beam or plane divergent beam), several as yet unsolved problems impede practical use of the above non-contact optical reading methods.
The first problem results from the diverse sizes of spectacle frames, which impose a minimum depth of field of the order of 4 cm. The constraints for satisfactory crimping of the lens into the bezel, in particular in metal frames, impose a relatively high level of accuracy for grinding the outside edge of the lens in corresponding relationship to the shape of the bezel. The resulting accuracy required in reading the shape of the bezel is of the order of one hundredth of a millimeter.
Because these two requirements, relating on the one hand to the depth of field and on the other hand to the accuracy of the measurement, are mutually contradictory, it is not possible at present to satisfy them with components available off the shelf. A compromise consisting of a depth of field of 4 cm for an accuracy of 0.01 mm imposes the use of a sensor providing 4000 measuring points, i.e. a CCD camera with 2000xc3x972000 pixels and a resolution of 0.5 pixel. This resolution is difficult to achieve because a large area camera implies a wide field, which constitutes a source of optical aberrations, especially at the edges, and this impedes obtaining a resolution of less than one pixel unless relatively costly and bulky high-performance optics are used. Furthermore, it is not easy to generate a divergent plane light beam that is sufficiently thin to be contained within a depth of 4 cm, which requires the use of relatively costly and bulky precision optics.
Given the above constraints, the only practical solution to the problem of implementing the above reading method with sufficient accuracy is to control the combination of the laser beam and the sensor mechanically to maintain it at a very small distance from the bezel, in order to restrict the depth of field required of the sensor. However, this requires a tracking mechanism whose complexity and cost are added to those of the optical reader device.
Optical methods of reading a bezel of a spectacle frame run into a second difficulty. Spectacle frames can be made with very different shapes and from diverse materials having their own optical properties, in particular with regard to reflection, absorption, diffusion and back-scattering. Thus an optical reading method can be validated only on condition that it proves to be effective for all types of frame, in particular those having rims of circular, oval or rectangular shape and made from metal, opaque plastics material, multicolored plastics material or transparent or translucent plastics material, with or without metal inserts. Because of this diversity of shapes and materials, reception by the optical sensor of the image of the trace of the laser beam on the bezel can be rendered impossible by certain optical phenomena depending on the illumination and image capture configuration. In particular, it is necessary to envisage the following difficulties:
a specific point or area of excessively intense reflection for a given reading illumination angle,
the presence of metal inserts in a transparent or translucent plastics material frame rim causing unwanted reflections masking the trace of the plane light beam on the bezel or making it unusable,
an angle of incidence of the light beam producing too much or too little reflection to be usable by the optical sensor, depending on the material from which the frame concerned is made, and
a pronounced oblong shape of the rim of the frame preventing illumination of certain sectors of the bezel at a given angle, the light beam impinging on the outside of the rim of the frame.
In the above context, one aim of the present invention is to provide an optical non-contact method of reading the three-dimensional shape of a profile, such as the bezel of a spectacle frame rim, avoiding the compromise between resolution and depth of field as much as possible by xe2x80x9cincreasingxe2x80x9d the depth of field, whilst retaining a satisfactory resolution, without it being necessary to use complex and costly optical reader means.
Another object of the invention is to minimize luminous signal losses, i.e. to render the image of the trace of the laser beam on the bezel usable by the optical receiver means for the greatest possible variety of situations and types of frame, in particular those previously referred to.
With a view to achieving at least one of the above two objects, the invention proposes an optical non-contact method of reading the three-dimensional shape of any profile in accordance with the luminous section principle, the method consisting of:
scanning the profile with a plane light beam intersecting the profile transversely,
simultaneously reading the trace of the plane light beam on the profile by means of optical receiver means having an optical pointing axis at a constant non-zero pointing angle to the light beam at a series of positions along the profile, and
deducing the three-dimensional shape of the profile from the readings effected at these various positions,
in which method, on each reading, the light beam whose trace on the profile is read by the optical receiver means is chosen from a plurality of predefined light beams which can be activated alternately.
To be more specific, the above method can advantageously be applied to the situation more particularly referred to hereinabove of reading the shape of the inside edge of a spectacle frame rim, referred to as the bezel.
There is also proposed, for implementing the above method, an optical non-contact device for reading the three-dimensional shape of the inside edge, referred to as the bezel, of a rim of a spectacle frame, the device including a support for the frame rim and a read head rotatable relative to the support about a rotation axis and associated with a sensor responsive to its angular position relative to the support, the read head including emitter means adapted to project a plane light beam intersecting the bezel transversely and optical receiver means adapted, regardless of the angular position of the read head relative to the frame support, to read the trace of the plane light beam on the bezel along an optical pointing axis at a constant non-zero pointing angle to the light beam, in which device the emitter means are adapted to project at least two separate light beams.
Accordingly, to capture each image, it is possible to choose, from among the plurality of light beams available, the one whose trace on the profile is the most propitious to accurate reading by the optical receiver means along their optical pointing axis. This choice of the most suitable light beam for the configuration encountered can be directed to achieving either or both of the two objects previously cited.
In a first aspect, the choice offered by the emitter means between a plurality of light beams can generate an artificial xe2x80x9cincreasexe2x80x9d in the depth of field of the optical receiver means, with no loss of accuracy. As previously mentioned, to preserve good reading accuracy without using complex, bulky and costly optical means, it is necessary to accept a restricted depth of field of the optical receiver means. It follows that with a single light beam, the trace of the beam could not fail to leave the field of the optical receiver means in various areas of the profile, unless the profile were tracked at a constant distance by the optical receiver means. According to the invention, this problem relating to the necessary narrowness of the field of the optical receiver means is compensated by the possibility of illuminating the profile with another light beam, offset relative to its inoperative counterpart, and thereby defining another range of reception in which its trace on the profile (bezel) is totally within the field of the optical receiver means. In other words, the various light beams define, in relation with the depth of field of the optical receiver means, different reading ranges which, on being juxtaposed, are additive and thereby define an overall reading range that is much more extensive than only one of those ranges taken in isolation. When scanning the profile, the light beams are activated alternately so that the trace of the active light beam is in the field of the optical receiver means. This choice between a plurality of beams offered by the emitter means thereby artificially demultiplies the depth of field, and it is therefore possible to use high-precision optical receiver means with a small depth of field, with no costly optical system that is difficult to implement.
In a second aspect, the choice between a plurality of light beams offered by the emitter means overcomes the reception difficulties resulting from the diverse shapes and materials of the frames (or other profiles). To capture each image it is possible to change the active light beam as soon as reading becomes impossible or inaccurate with the light beam that is initially active. Clearly, the problems previously cited, relating to the difficulty or impossibility of receiving images or even of illuminating particular areas of the profile, arise only in a relatively precise geometrical configuration of the light beam and the optical receiver means relative to the area concerned of the profile. Consequently, the reception or illumination problems that arise with a particular light beam will generally not arise with another light beam having a different geometrical configuration. This is why alternating between a plurality of light beams overcomes the reception or illumination difficulty in most cases.
To improve reading accuracy, it is preferable to have a pointing angle greater than 45xc2x0. In practice, a pointing angle of approximately 70xc2x0 is a good compromise for highly accurate and effective reading in most configurations.
The invention provides a number of additional advantageous features contributing to achieving either or both of the two technical effects developed hereinabove.
In particular, to increase the depth of field of the optical receiver means artificially, the plurality of light beams can include at least one series of at least two juxtaposed and substantially parallel light beams. In practice, a number of juxtaposed light beams from three to eight in the or each series is satisfactory for reading most frame bezels.
Accordingly, when scanning the profile, if the depth of field of the optical receiver means is reduced to the benefit of reading accuracy, the trace of the active light beam may escape from the receive field. In this case, it is sufficient to deactivate that light beam and to activate the adjacent light beam, which, because it is offset relative to initial beam, covers a receive range offset relative to that covered by the initially active beam and therefore illuminates the profile (bezel) along a trace which is within the receive field of the optical receiver means. Accordingly, depending on the distance between the area concerned of the profile (bezel) and optical receiver means (to be more precise the rotation axis of the read head), the choice can be made to activate the parallel juxtaposed light beam covering the receive range corresponding to that distance and whose trace is in the receive field of the optical receiver means. As previously explained, this artificially increases the receive depth of field by creating between the parallel juxtaposed light beams reading ranges corresponding to a particular range of the distance of the profile from the optical receiver means. In other words, the depth of field needed to read a profile in an overall range is divided by the number of parallel juxtaposed light beams, the necessary overall receive range being divided into the same number of small reading ranges, respectively associated with various parallel juxtaposed beams.
Instead of, or in combination with, the plurality of juxtaposed light beams, to overcome the reception problems resulting from the diverse shapes and materials of the frames in particular areas of the bezel of the frame rim, or more generally of the profile concerned, the plurality of light beams can include at least two light beams with an angle between them greater than 20xc2x0.
Accordingly, a given area of the profile (bezel) can be illuminated alternately by one or the other of these two beams at two different angles of incidence. Then, if the illumination of a given area of the profile by one of the light beams cannot produce a satisfactory reading, for any of the reasons previously explained, that beam is deactivated in favor of an angularly offset beam, to illuminate the area concerned of the profile at another angle with a very high probability of producing a satisfactory reading, or at least a better reading than the first beam. Similarly, if, in a particular area of the profile, to be more specific in a particular area of the bezel of a frame rim, the light beam is reflected by a metal insert buried in a translucent plastics material, causing abnormal reflection of the beam, preventing accurate reading of its trace, it is again sufficient to deactivate the beam encountering the metal insert and activate the other, angularly offset beam. The latter, illuminating the profile at a different angle, will not encounter the metal insert and the unwanted reflection from that insert will therefore be eliminated, so that a reading can be obtained under the normal conditions.
In one advantageous configuration the plurality of light beams includes at least one pair of light beams symmetrical to each other with respect to a median reading plane containing the optical pointing axis. Accordingly, if the frame rim to be measured is oblong in shape, and the initially active beam no longer illuminates the bezel from the inside of the rim, but to the contrary illuminates the outside of the rim, it is sufficient to activate the symmetrical beam which, being on the inside of the rim, will illuminate the bezel correctly. It is therefore possible in particular to provide that each of the two symmetrical beams is at a pointing angle to the optical pointing axis of approximately 70xc2x0 to obtain maximum reading accuracy and efficacy.
In one preferred embodiment, combining the choice between a plurality of parallel juxtaposed light beams and a plurality of angularly offset light beams, the plurality of light beams could include a first series of at least two parallel juxtaposed light beams and a second series of at least two parallel juxtaposed light beams, the beams of the two series being symmetrical in pairs with respect to the median reading plane.
In an advantageous embodiment, each plane light beam is derived from a rectilinear coherent light beam on whose trajectory is placed a spreader lens causing the rectilinear coherent light beam to diverge in the required plane transverse to the bezel.
To be more precise, each rectilinear coherent light beam is emitted by its own laser source which is disposed vertically, parallel to the rotation axis of the read head, and an oblique mirror is disposed on the trajectory of the rectilinear coherent light beam, upstream of the spreader lens, to reorient the trajectory of the rectilinear coherent light beam, and consequently that of the divergent plane light beam derived therefrom, toward the bezel of the rim of the frame carried by the support. This vertical disposition of the laser sources considerably reduces the overall size of the read head in the radial direction, i.e. its overall size in the direction perpendicular to its rotation axis.
Similarly, to reduce the overall axial size of the read head, the optical receiver means include a matrix optical sensor and an associated objective lens both placed on a common optical axis and at least one mirror is placed obliquely to the optical axis to reorient it along a broken-line path whose final branch forms the required optical pointing axis.
For example, the optical axis is oblique to the rotation axis of the read head and at least two mirrors are placed on respective opposite sides of the axis of the read head to reorient the optical axis along a zig-zag path with at least three branches.
Finally, for rapid and convenient reading of both rims of the same frame, the read head is rotatably mounted on a carriage that can slide relative to the support for the frame between two reading positions in which the read head alternately faces one or the other of the two rims of the frame.
On the other hand, the various materials (metal, opaque or translucent plastics material, etc.) from which frames are made having very different optical properties, in particular with regard to their reflectivity characteristics, the power of the light beam can advantageously be regulated as a function of the luminous flux received by the optical receiver means to maintain that flux at a substantially constant level suited to the capabilities of the receiver means for satisfactory reading accuracy regardless of the type of frame.
Finally, for reading the shape of the outside edge of a glass template, the device advantageously includes additional support means for a template adapted to be turned relative to the read head about an axis parallel to the axis of the read head by associated drive means to move the outside edge of the template in front of the emitter means and the optical receiver means while the read head is held fixed in a particular angular position.
Other features and advantages of the invention will become apparent on reading the following description of one particular embodiment of the invention, which description is given by way of non-limiting example only and with reference to the accompanying drawings.